Do not understand collect method use wit property


#1

Hi all,
Can someone explain to me in terms of ruby why this works?

Story.find_all_by_user_id(2).collect(&:link) # 1 method call

This is in ROR console. link is a property of Story. The above command
finds all db rows and all their properties (columns) with user_id == 2
and creates an array with only the link property.

This is same as:
Story.find_all_by_user_id(2).collect{|x| x.link} # 2 block

I understand function #2 with use of a block.

I do not understand function #1 in terms of syntax. What is &:link? I
know it represents a property and could accept that this is the way it
works but I want to understand this in terms of ruby syntax. Is &:link
a block? If so it would have to represent x.link. I do not understand
how this is generated in ruby.

TIA,
Pete


#2

They are both equivalent in terms of functionality.

The first is a common rails idiom:

@some_collection.collect(&:id)
or
@some_collection.map(&:id) #I’ve seen this one used more than
collect…

The ampersand syntax is simply telling the interpreter that the symbol
is the block parameter to the method. Internally the to_proc method of
the symbol class is called.

Here’s a reference:
http://apidock.com/rails/Symbol/to_proc

Beware, doing @foo.map(&:id) doesn’t perform as well as @foo.map { |f|
f.id } for big numbers of foos…
Also note that this is a Rails only feature (ie: try it on plain irb
and it will not work).

On Mar 11, 1:35 pm, Hiro P. <rails-mailing-l…@andreas-


#3

Cool. Thanks for explanation.

Pete

Harold wrote:

They are both equivalent in terms of functionality.

The first is a common rails idiom:

@some_collection.collect(&:id)
or
@some_collection.map(&:id) #I’ve seen this one used more than
collect…

The ampersand syntax is simply telling the interpreter that the symbol
is the block parameter to the method. Internally the to_proc method of
the symbol class is called.

Here’s a reference:
http://apidock.com/rails/Symbol/to_proc

Beware, doing @foo.map(&:id) doesn’t perform as well as @foo.map { |f|
f.id } for big numbers of foos…
Also note that this is a Rails only feature (ie: try it on plain irb
and it will not work).

On Mar 11, 1:35�pm, Hiro P. <rails-mailing-l…@andreas-